The present invention relates to a method of creating a photometric measurement path-length flow cell and more particularly, to provide an adjustable path-length in a flow cell.
The measurement of an analyte of interest by a photometric detector is dependent upon several parameters for the accurate analysis of the object of detection. The path-length of the object of interest through the flow cell is of importance in the accuracy and sensitivity of analysis. The determination of the accurate measurement of that path-length becomes essential in devices that have adjustable path-lengths. Several approaches to the determination of path-length in the prior art have been attempted with certain limitations.
For example, the Sonn-Tek adjustable flow cell, available from Sonntek, Inc. of New Jersey, and illustrated in FIG. 1, is configured to adjust for the desired path-length by the movement of one or two small glass rods by the mechanical movement of special screws that exert pressure upon the glass rods to reduce the path-length. The mechanism of increasing the path-length in the Sonn-Tek flow cell is by the internal cell pressure of the cell. Internal cell pressures of 250 psi or more are needed in order to back the above glass rod out when an increase in path-length is needed. A major disadvantage of this method of adjusting the path-length is that the user of the device has only an approximate indication of the measurement path-length. To calculate the true path-length, the user must iteratively rely on chemistries until they are confident that the flow cell has been adjusted correctly. An additional problem with this adjustable cell is the potential for contamination from unswept volumes due to the sealing mechanisms typically used on such cells. While this contamination problem may not be a major issue at high preparative flow rates, it becomes increasingly problematic at flow rates that are typical of analytical work. Another limitation of this device is that this type of flow cell is generally more difficult to rebuild and maintain than the standard non-adjustable cells.
Another attempt, illustrated in FIG. 2, has been to fashion flow cells where the desired critical measurement of the optical path-length and the fluidic path-length are the same and inherent with the design of the cell body. A severe limitation imposed by this approach is that each path-length requirement would need a different flow cell body to be machined. The fabrication of these short path-length flow cells are relatively expensive to machine. Additionally, for measurement path-length requirements that are shorter than approximately 1.0 mm, conventional machining methods become unreliable due to the thin cross sections involved. The fluidic connections are also problematic when the path-length is less than approximately 1.0 mm. That is, it is difficult getting a 1.0 mm internal diameter tubing to work with a 0.5 mm path-length cell without flow restriction.
Known implementations suffer limitations with respect to reliability, expense, sensitivity and accuracy.
The present invention provides a photometric measurement flow cell having measurement path-lengths that can be reliably, accurately, and inexpensively adjusted down to less than 0.1 mm.
According to the invention, path-length is controlled in a common flow cell body by dimensional parameters of a stepped sealing optical element. The stepped optical element of the present invention is made of an optical glass, which in the illustrative embodiment is a fused silica glass. The stepped optical element includes a stem portion that can be made in various lengths and utilized to create a family of flow cell measurement path-lengths. The replacement of one stepped element with another having a different stem length within the flow cell creates a reliable method to adjust the measurement path-length of the flow cell.
The adjustable path-length of the flow cell of the present invention provides many benefits over conventional adjustable path-length flow cells. The flow cell configured according to the present invention is no more difficult to rebuild and maintain than conventional analytical flow cells. Bandspreading is reduced when using the present invention at low flow rates, compared to the conventional adjustable path-length flow cells. The reliability of the measurement path-length is greatly increased. The potential for contamination from unswept volumes due to the conventional sealing methods in adjustable path-length flow cells is eliminated. The lack of complexity in the manufacturing of the adjustable path-length flow cell of the present invention greatly reduces its cost. The machining problems and complexities associated with conventional adjustable flow cell for path-lengths below 1.0 mm are avoided.